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Ichnology of the Cardium Formation (Pembina Oilfield): Implications for Depositional and Sequence Stratigraphic Interpretations
The Cardium Formation is one of the most important oil and gas producing formations in the province of Alberta. In the Pembina oilfield, the availability of abundant drill core from the Cardium Formation has allowed investigation of “muddy” strata which would not be as well preserved in outcrop. The lithofacies and associated ichnogenera identified in this study are useful in environmental interpretation and may help in the application of sequence stratigraphy to the Cardium Formation.
The Cardium Formation is late Turonian in age, and was deposited in the epeiric seaway which extended across North America at that time. Within the formation, coarsening- and thickening-upward sequences of mudstone and sandstone are unconformably overlain by conglomerate and mudstone. Ichnofossils are found in abundance in all cores examined. The relationships of these ichnofossils to each other and to the lithofacies of the Cardium Formation, provide evidence for an interpretation of storm-dominated deposition in a proximal offshore setting. Relative sea-level variations are documented in the multiple coarsening-upward sediment packages and by an unconformity surface found at the base of the conglomerate. The unconformity surface at the base of the conglomerate may represent a sequence boundary and may be indicative of a maximum lowstand of sea level.
The base of the conglomerate also contains a sparsely developed yet highly specialized suite of ichnofossils which was emplaced when sediments were firm but unconsolidated. These ichnofossils are indicative of marine to marginal marine conditions which developed after erosion had exhumed a previously buried substrate. Recognition of this firmground ichnofossil suite may prove useful for other investigations in sequence stratigraphy.
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